In recent years, air pollution by NOx or floating small particulates has developed in large cities. As for the small particulates floating in the air, 20 to 30% of them are said to be produced by black smoke exhausted by Diesel engines of automobiles. As a counter-measure against the black smoke, there is provided an exhaust gas cleaner which, placed in the exhausting system, comprises a filter to trap small particulates in the exhaust gas and a refreshing apparatus to burn the trapped particulates.
Among the conventional filters, a ceramic monolithic type is known as is disclosed in U.S. Pat. No. 4,364,761, which is incorporated herein by reference. This conventional ceramic monolithic filter comprises many long cells arranged side-by-side to form a honeycomb structure, one cell having an inlet at one end thereof and being plugged at the other end, and an adjacent cell being plugged at the same end as the inlet of the first cell but having an outlet at the other end. When the exhaust gas enters the inlet and passes along the porous wall between the cells, the particulate in the gas is trapped. If the amount of trapped particulate increases, the porous walls are choked, of course, by the particulates. This results in an increase of back pressure of the engine. Accordingly, it is necessary, when the trapped particulates exceeded a certain amount, to remove the particulates to suppress the load increase on the engine due to increased back pressure. The particulates consist of solid carbon and soluble organic fraction (SOF) which is soluble in an organic solvent such as dichloromethane, both being combustible and burned if heated over 600.degree. C., although the temperature somewhat varies depending upon the kind or loading condition of the engine. Thus, the exhaust gas cleaner is provided with a filter regenerating apparatus such as an electric heater or gas burner. For the exhaust gas filter, it is necessary that it not be choked or molten when heated for regenerating, and, usually is a porous material of large porosity and high melting temperature. For the exhaust gas filter to meet the above described requirement, U.S. Pat. No. 4,652,286 which is incorporated herein by reference discloses a filter which is made by sintering a mixture of silica-alumina fiber and silica-alumina clay at 1200.degree. C. to make a sheet assembled in a honeycomb structure. This filter, however, after repeated particulate trappings and heating regenerating, produced cracks in the wall, and is therefore not satisfactory.